Small fixes in multiple views

Custom population

• Fixed bug: Using filters made the notification appear saying the excluded student numbers are forbidden or invalid.
• Also added notification in cases where there were 0 valid student numbers in the search

Student statistics

• Fixed bug: courses were incorrectly marked as HOPS or not HOPS based on whether they were prefixed with 'AY' or not
• Fixed bug: Study plan was selected wrongly when multiple programmes existed in the same studyright

Students pane > General tab

• Show HOPS-credits for licentiates also (combined programmes)

Miscellaneous

• Fixed calculation of HOPS-credits in updater when attainment has 'AY' code
• Multiple internal fixes, like adding tests, removing dead code and fixing key- & proptype-issues

Faculty

• Modified info text describing credits
• Fixed logic for computing credits